Making and using an ultra-thin copper foil

ABSTRACT

This invention provides a composite foil comprising an organic release layer between a metal carrier layer and an ultra-thin copper foil, and a process for producing such composite foils comprising the steps of depositing the organic release layer on the metal carrier layer and then forming an ultra-thin copper foil layer on said organic release layer, preferably by electrodeposition. The organic release layer preferably is a heterocyclic compound selected from triazoles, thiazoles, imidazoles, or their derivatives, and provides a uniform bond strength which is adequate to prevent separation of the carrier and ultra-thin copper foil during handling and lamination, but which is significantly lower than the peel strength of a copper/substrate bond, so that the carrier can easily be removed after lamination of the composite foil to an insulating substrate. The invention also includes laminates made from such composite foils and printed wiring boards made from such laminates.

FIELD OF THE INVENTION

[0001] The present invention relates generally to making and usingultra-thin copper foil and, more particularly, to a composite foil whichfacilitates handling of ultra-thin copper foil in the production ofprinted wiring boards. The invention also relates to a composite foil inwhich an organic release layer with uniform bonding strength is disposedbetween a metallic carrier and an ultra-thin copper foil, and theprocesses of producing and using such a composite foil.

BACKGROUND OF THE INVENTION

[0002] With the recent miniaturization and densification of electronicequipment, the pattern widths and spacing of printed wiring boards havebecome smaller. As a result, the copper foil used has tended to becomethinner, for example, 12 μm thick foils, rather than conventional 35 μmand 18 μm foils. Recently, the need for thin copper foil has increasedand ultra-thin copper foils have been tried. Handling a copper foil of12 μm or less, however, is difficult. For example, it can wrinkle ortear while being produced and/or handled. Similar problems occur when anultra-thin copper foil is used as the outer layer of a multi-layerprinted wiring board. A method of preventing these problems withhandling ultra-thin copper foil is needed.

[0003] It has been previously proposed to support ultra-thin copperfoils on a metal carrier layer to allow the copper foil to be separatedfrom the carrier layer. Several carrier metals and types of releaselayers have been suggested. Producing printed wiring boards from suchsupported ultra-thin copper foils could be done by electrolyticdeposition of a copper layer having a thickness of 1-12 μm onto a metalcarrier layer having a thickness of 18-70 μm, then applying the surfaceof the copper layer to a prepreg, such as a glass-reinforced epoxy resinor the like, and laminating by hot pressing. Finally, the metal carrierlayer would be separated, leaving a copper-clad laminate from which aprinted wiring board can be made.

[0004] When a copper sheet is used as a carrier, a chromium layer may beused as a release layer between the copper foil and the copper carrierlayer, as has been disclosed in, for example, Japanese PatentApplication Publication (Examined) No. Sho 53-18329.

[0005] Alternatively, when aluminum is used as the carrier layer,several types of release layers have been proposed, for example:

[0006] 1. a release layer of the sulfides or oxides of Cr, Pb, Ni and Ag(for example, in U.S. Pat. No. 3,998,601);

[0007] 2. a release layer formed of nickel or nickel alloy plating afterzinc immersion (for example, in U.S. Pat. No. 3,936,548);

[0008] 3. a release layer of aluminum oxide (for example, in JapanesePatent Application Publication (Examined) No. Sho 60-31915 and U.K.Patent No. GB 1,458,260); or

[0009] 4. a release layer of silica (for example, in U.S. Pat. No.4,357,395).

[0010] Such conventional supported copper foils have, however, beenfound to present problems.

[0011] Since the release layer is not uniform over the surface of thecarrier layer, bond strength between the carrier layer and theultra-thin copper foil is uneven. Consequently, when the carrier layeris peeled off after laminating a composite foil, some of the ultra-thincopper foil may remain on the carrier layer or some of the carrier layermay remain on the ultra-thin copper foil. In either case, the requiredcircuit pattern cannot be made. Further, weak bond strength may cause anultra-thin copper foil to partially or entirely separate from thecarrier layer during production and use of the composite foil.

[0012] When oxides, sulfides, chromium or inorganic materials, such aschromium or the like, are used as release layers, some of the inorganicmaterial remains on the surface of the ultra-thin copper foil after thecarrier layer is peeled off. This inorganic material must be removedbefore the circuit patterns are made, making it necessary to add extraprocessing steps.

[0013] Finally, when the composite foil is laminated to a substrate,such as an epoxy prepreg at high temperatures, it becomes difficult topeel off the carrier layer.

[0014] Because of these problems, composites of ultra-thin copper foilon a support layer are not generally used in industry at present,despite the proposed methods just discussed.

[0015] Accordingly, an object of the present invention is to provide acomposite foil which overcomes the problems discussed above and aprocess for making such composite foils. Another object of the presentinvention is to provide a copper-clad laminate which is made by usingsuch a composite foil, and a printed wiring board employing such acopper-clad laminate.

[0016] The inventors have investigated the metals and/or metal compoundswhich have conventionally been suggested as release layers for compositefoils in the prior art. They have found that when peeling off a supportlayer from an ultra-thin copper foil after the copper foil has beenlaminated to a resin substrate by hot pressing, the peel strength isvariable and the bond can be too strong. Also, when a release layer isnot uniformly formed, or when heat is used during laminating, the metalused in the release layer may be diffused into both the support layerand the ultra-thin copper foil.

[0017] The inventors have also investigated organic compounds which havenot been considered as release layers. As a result, the inventors havefound that the above-mentioned problems of composite foils can be solvedby using certain organic compounds as release layers, as will be seen inthe discussion below.

SUMMARY OF THE INVENTION

[0018] As used in this invention, the term “bond strength” refers to theforce required to separate the carrier layer from the ultra-thin copperfoil. The term “peel strength” refers to the force required to separatethe ultra-thin copper foil from a substrate to which it has beenlaminated.

[0019] The present invention comprises a composite foil with a newrelease layer having the following features.

[0020] 1. The release layer is easy to apply.

[0021] 2. The bond strength between the ultra-thin copper foil and thecarrier layer is uniform, and has a relatively low value compared to thepeel strength of the copper foil after lamination to a substrate.

[0022] 3. Mechanical polishing and pickling to remove an inorganicmaterial which remains on the surface of an ultra-thin copper foil isnot necessary since no inorganic material is used. Thus, the formationof wiring patterns is simplified by reducing the number of processingsteps.

[0023] 4 Although bond strength is low, it is sufficient to preventseparation of the ultra-thin copper foil from the carrier layer duringhandling.

[0024] 5. The composite foil has sufficient peel strength afterlamination to a substrate and does not separate during processing into aprinted circuit board.

[0025] 6. The carrier layer can be separated from the ultra-thin copperfoil even after laminating at elevated temperatures.

[0026] 7. It is easy to recycle the carrier layer after it has beenseparated from since the residual release layer is easy to remove.

[0027] Organic compounds which have been found useful in makingcomposite foils have the following characteristics.

[0028] 1. They are organic compounds which contain nitrogen, preferablyheterocyclic compounds.

[0029] 2. They form a bond with copper.

[0030] 3. They retain the ability to bond with copper even afterexposure to the temperatures used in laminating copper foil to aninsulating substrate, preferably at a temperature of 120° C. or higher.

[0031] 4. They form a bond between the ultra-thin copper foil and thecarrier layer which is relatively low compared to the peel strengthbetween the ultra-thin copper foil and the insulating substrate. Thebond strength between two layers of copper is sufficient to preventtheir separation during handling and lamination, but low enough topermit the carrier layer to easily be removed after the composite foilhas been laminated.

[0032] 5. They provide a very thin release layer which permits uniformelectrodeposition of the copper foil.

[0033] Preferred organic compounds are heterocyclic compounds containingnitrogen, in particular, members of the group consisting of triazoles,thiazoles, and imidazoles, or their derivatives. Similar materials havebeen used for corrosion protection of copper, but they have not beensuggested as being suitable for release layers.

[0034] In one aspect, the invention is a composite foil having anorganic release layer having the characteristics just described, whichis disposed between a metal carrier layer, which may also be a foil, andan ultra-thin copper foil.

[0035] The invention also includes a process for producing a compositefoil by the steps of forming an organic release layer on a metal carrierlayer and thereafter forming an ultra-thin copper foil on the organicrelease layer, preferably by electrodeposition.

[0036] A copper-clad laminate of the present invention comprises thecomposite foil according to the invention laminated to an insulatingsubstrate. Alternatively, it comprises the ultra-thin copper foillaminated to the insulating substrate which remains after peeling offthe metal carrier layer.

[0037] A printed wiring board of the present invention results fromforming a wiring pattern from the ultra-thin copper foil exposed byseparating the metal carrier layer from the copper-clad laminate justdescribed.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0038] In general, the composite foil of the present invention ischaracterized by having an organic release layer between a metal carrierlayer and an ultra-thin copper foil. The organic release layer satisfiesthe five characteristics discussed above. The release layer is anorganic compound containing nitrogen and, preferably, is a heterocycliccompound selected from the group consisting of triazoles, thiazoles, andimidazoles, or their derivatives.

[0039] As the metallic carrier layer, copper or a copper alloy ispreferably used since the release layer bonds to copper. An organiccompound according to the present invention may be applied to a supportmaterial other than copper or a copper alloy, for example, copper-platedaluminum. Other metals could be used, provided the release layer forms auniform bond with that metal as well as with the ultra thin copper foil.The thickness of the supporting metallic layer is not considered to becritical, and it may be a foil from about 18-70 μm thick. Since atypical carrier layer is relatively thin, it will also be referred to asa foil, but it is to be understood that the support layer could bethicker than ordinary foils. For example, heavier carrier sheets up toabout 5 mm thick may be used.

[0040] The thickness of the ultra-thin copper foil layer to be formed onthe organic release layer is generally not more than 12 μm thick and maybe much thinner, for example, 5-7 μm or less. The reason for this isthat a copper foil having a thickness of more than 12 μm can be producedby conventional processes, and can be handled without a carrier layer.The ultra-thin copper foil layer is preferably formed byelectrodeposition, although vapor deposition or electroless plating maybe employed, provided that the ultra-thin copper layer is suitable formaking circuit patterns and has acceptable bond and peel strengths.

[0041] In a preferred embodiment, the organic release layer comprises atleast one member selected from the group consisting of triazoles,thiazoles, and imidazoles, or their derivatives, which are selected fortheir ability to bond to copper and to retain the ability to bond tocopper at lamination temperatures. The triazole group includesorthotriazole (1,2,3-triazole) and isomers thereof, or derivativesthereof Orthotriazole derivatives include benzotriazole, tolyltriazole,carboxybenzotriazole, chlorine substituted benzotriazole, aminotriazoleand isomers thereof, or derivatives such as alkali metal salts or aminesalts and the like. As the isomers of the aminotriazole,3-amino-1,2,4-triazole, 2-amino-1,3,4-triazole, 4-amino-1,2,4-triazoleand 1-amino-1,3,4-triazole can be used. Examples of derivatives ofaminotriazole include sodium salts or amine salts including, forexample, monoethanolamine salts, cyclohexylamine salts, diisopropylaminesalts, morpholine salts and the like.

[0042] Examples of thiazoles and derivatives thereof include thiazole,2-mercaptobenzothiazole, dibenzothiazyldisulfide, cyclohexylamine saltsof 2-mercaptobenzothiazole, dicyclohexylamine salts of2-mercaptobenzothiazole and the like.

[0043] Examples of imidazoles and derivatives thereof include imidazole,2-methylimidazole, 2-ethyl-4-methylimidazole, 2-phenylimidazole,1-benzyl-2-methylimidazole and the like. The organic release layeraccording to the present invention may be formed not only of theabove-mentioned organic compounds, but also of related organic metalcompounds such as the zinc salt of 2-mercaptobenzothiazole. Since thequantity of metal atoms is small in such compounds, they can bedistinguished from the metals and inorganic compounds previously used inthe art.

[0044] Where adhesion is measured in accordance with the Japanesestandard JIS-C-6481, the range of the bond strength between theultra-thin copper foil and the carrier of the composite foils of theinvention is about 0.005-0.3 kgf/cm, preferably 0.005-0.1 kgf/cm, toassure that the carrier layer can be separated from the ultra-thincopper foil after laminating the composite foil on an insulatingsubstrate. When less than 0.005 kgf/cm, the bond strength is so weakthat bulging or separating of the ultra-thin copper foil occurs duringlamination with a substrate or during punching or drilling of a laminateor a circuit board. When greater than 0.3 kgf/cm, the bond strength isso large that it is necessary to provide a special treatment whenseparating the metal carrier layer as, for example, the use of anaqueous medium in U.S. Pat. No. 3,886,022.

[0045] A composite foil of the invention has little or no variability inthe bond strength between the metal carrier layer and the ultra-thincopper foil. The bond strength is consistent both across individualcomposite foils and among multiple examples of the composite foil.

[0046] Since only a thin organic film is attached to the surface of anultra-thin copper foil after separating the supporting metallic layer,removal of the organic film is possible by cleaning with a dilute acidsolution. A harsh pickling step is not necessary. In addition to this,the organic layer remaining on the surface of the ultra-thin copper foilafter the carrier has been removed can serve as a passivator to preventoxidation. Furthermore, since it is easy to recycle a supportingmetallic layer after it has been separated, the production andenvironmental problems discussed above are avoided.

[0047] In producing a composite foil of the invention, an organicrelease layer is formed on a metal carrier layer, and thereafter anultra-thin copper layer is deposited on the organic release layer. Priorto the formation of the organic release layer, it is preferable toremove any copper oxide on the surface of the metal carrier layer toassure that a uniform bond strength is achieved. This may be done, forexample, by rinsing the carrier in a dilute acid solution, e.g., dilutesulfuric acid. The organic release layer may be applied to the metalcarrier layer by dipping or coating, or any method by which a uniformlayer is formed on a carrier. For example, by dipping a metal carrierlayer into an aqueous solution of the selected organic compound, e.g., atriazole derivative, to form an organic release layer. Concentration ofthe organic compound in the aqueous solution is preferably about 0.5-10%by weight, and dipping time is preferably about 5-60 seconds. Althoughhigher concentrations and longer dipping times are possible, they arenot preferred since they would increase costs and decrease productivity.After removing the carrier from the solution, the excess is removed byrinsing with water so that only a very thin layer bonded to the carriersurface remains. The thickness of the film after rinsing is difficult tomeasure accurately, but it is believed to be about 30-60 Å. The range ofacceptable film thicknesses is not known, but if the film is too thin,the ultra-thin copper layer will adhere to the carrier, while if thefilm is too thick, uniform electrodeposition of copper is not feasible.

[0048] The ultra-thin copper foil preferably is electrodeposited on topof the organic release layer which has been placed on the metal carrier.There are several methods of electrodepositing copper, such as using acopper pyrophosphate plating bath or an acidic copper sulfate platingbath, among others. While any type of plating bath may be used to formthe ultra-thin copper foil, some types are preferred. To avoidundesirable pin holes and/or porosity, initial plating is preferablydone in a copper cyanide bath or a copper pyrophosphate bath, which givemore uniform electrodeposition of copper. Copper pyrophosphate platingis preferred since it has advantages with respect to the environment andsafety of operations. On the other hand, copper sulfate plating bathsare advantageous if one considers productivity and cost. The acidic bathmay, however, remove the thin release layer and, thus, the initialplating in an acidic bath should be avoided for achieving best results.Thus, it is preferred that a first copper plating step providing athickness of at least 0.5 μm, preferably 0.5-1.0 μm, is done in a copperpyrophosphate bath, followed by a second plating step using a coppersulfate plating bath to provide the desired thickness of the ultra-thincopper foil. This method is used in the Examples below.

[0049] The conditions for copper pyrophosphate plating are not believedto be critical. It is preferred, however, that the copper concentrationin the copper pyrophosphate plating bath be about 10-50 g/L and thepotassium pyrophosphate about 100-700 g/L. The pH of the electrolyticsolution preferably should be about 7-12. The bath temperature should beabout 30-60° C., and the current density about 1-10 A/dm².

[0050] The conditions for acidic copper sulfate plating are also notconsidered critical. It is preferred, however, that the copperconcentration in the copper sulfate plating bath be about 30-100 g/L,and that the sulfuric acid be about 50-200 g/L. The bath temperature ofelectrolytic solution preferably is about 30-80° C. and the currentdensity about 10-100 A/dm².

[0051] In order to enhance adhesion of the ultra-thin copper foil to aninsulating substrate, a bond enhancing treatment may be placed on theultra-thin copper foil layer by conventional methods, such as forming anodular copper deposit on the surface of the foil by adjusting platingconditions. An example of a nodularization process may be found in U.S.Pat. No. 3,674,656. After the bond enhancing treatment is complete, aconventional passivation treatment may be added to the surface of theultra-thin copper foil, such as by depositing zinc, zinc chromate,nickel, tin, cobalt and chromium on the nodularized ultra-thin copperlayer. An example of such methods may be found in U.S. Pat. No.3,625,844.

[0052] The surface of the treated ultra-thin copper foil produced by themethods just described is laminated to an insulating substrate usingheat and pressure to obtain a copper-clad laminate. As the insulatingsubstrate, any of the resin substrates which are used to make circuitboards for electronic equipment can be used. Such resin substrates mayinclude FR-4 (glass-fiber-epoxy), paper-phenolic, paper-epoxy,polyimides and the like. The metal carrier layer is then separated,leaving a copper-clad laminate consisting of the ultra-thin copper foiland insulating substrate. A printed wiring board is made by formingwiring patterns from the exposed ultra-thin copper layer.

[0053] The present invention will be described in further detail withreference to Examples below.

EXAMPLE 1

[0054] As a metal carrier layer, an electrolytically deposited copperfoil having a thickness of 35 μm was made. Such foils typically have arough or matte side and a smooth or shiny side. Then, an organic releaselayer was formed on the shiny side of the foil, followed by a firstcopper plating step, a second copper plating step, a bond enhancingtreatment, and a passivation treatment, using the following procedures.

[0055] A. Formation of the Organic Release Layer

[0056] The 35 μm copper foil was dipped into 2 g/L carboxybenzotriazole(CBTA) aqueous solution at 30° C. for 30 seconds, removed, and rinsed indeionized water to form an organic release layer of CBTA.

[0057] B. First Copper Plating Step

[0058] A cathodic electroplating treatment was carried out with acurrent density of 3 A/dm² by using a pyrophosphate plating bath, with abath temperature of 50° C. and a pH of 8.5. The bath contained 17 g/L ofcopper and 500 g/L of potassium pyrophosphate. An ultra-thin copper foilhaving a thickness of 1 μm was deposited on the organic release layer.

[0059] C. Second Copper Plating Step

[0060] The surface of the 1 μm ultra-thin copper was rinsed in deionizedwater and then electroplated at a current density of 60 A/dm² using acopper sulfate plating bath having a bath temperature of 50° C., andcontaining 80 g/L copper and 150 g/L sulfuric acid, to deposit about 5μm thickness of copper, thereby obtaining an ultra-thin copper foilhaving a total thickness of about 6 μm.

[0061] D. Bond Enhancing Treatment

[0062] The bond enhancing treatment was carried out on the surface ofthe ultra-thin copper foil layer by a conventional nodularizing surfacetreatment using a copper sulfate plating bath. The current density wasincreased to form nodules of copper on the surface of the ultra-thincopper layer.

[0063] E. Passivation Treatment

[0064] A passivation treatment was applied to the surface of the bondenhanced ultra-thin copper foil layer by depositing zinc chromate fromsolution by conventional electrodeposition methods to obtain a compositefoil ready for lamination.

[0065] The composite foil was laminated on four sheets of 0.1 mm thickFR-4 prepregs, and then hot pressed under a pressure of 25 kg/cm² at175° C. for 60 minutes to obtain a copper-clad laminate. The strength ofthe bond between the ultra-thin copper foil and the 35 μm copper carrierlayer was measured in accordance with JIS-C-6481, and found to be 0.015kgf/cm. The copper carrier layer could easily be separated from thecopper-clad laminate, and the bond strength was found to be uniformacross the sample.

EXAMPLE 2

[0066] A composite foil was made in the same manner as in Example 1,except that an organic release layer was formed by using an aqueoussolution of 2 g/L N,N bis(benzotriazole-methyl) urea (BTD-U) instead ofcarboxybenzo-triazole (CBTA).

[0067] The composite foil was laminated on four sheets of 0.1 mm thickFR-4 prepregs, as in Example 1, using a pressure of 25 kg/cm², and atemperature of 140° C. for 60 minutes to obtain a copper-clad laminate.The strength of the bond between the ultra-thin copper foil and the 35μm copper carrier was measured to be 0.025 kgf/cm. The copper carriercould easily be separated from the copper-clad laminate, and the bondstrength was uniform across the sample. Then, the copper-clad laminatewas post-cured at 175° C. for 60 minutes.

EXAMPLE 3

[0068] A composite foil was made in the same manner as in Example 1,except that the organic release layer was formed by using an aqueoussolution of 2 g/L benzotriazole (BTA) instead of carboxybenzotriazole(CBTA).

[0069] The composite foil was laminated on four sheets of 0.1 mm thickFR-4 prepregs, as in Example 1, using a pressure of 25 kg/cm² at 140° C.for 60 minutes to obtain a copper-clad laminate. The strength of thebond between the ultra-thin copper foil and the 35 μm copper carrier wasmeasured to be 0.043 kgf/cm. The copper carrier could easily beseparated from the laminate, and the bond strength was uniform acrossthe sample. Thereafter, the laminate was heated at 175° C. for 60minutes to postcure it.

EXAMPLE 4

[0070] A composite foil was made as in Example 1, except that an organicrelease layer was formed by using a mixed aqueous solution of 2 g/Lcarboxybenzotriazole (CBTA) and 0.5 g/L benzotriazole (BTA) instead ofcarboxybenzotriazole (CBTA).

[0071] The composite foil was laminated on two sheets of polyimideprepregs, each having a thickness of 0.1 mm, using a pressure 25 kg/cm²at 216° C. for 270 minutes to obtain a copper-clad laminate. Thestrength of the bond between the ultra-thin copper foil and the 35 μmcopper carrier was measured to be 0.009 kgf/cm. The copper carrier couldeasily be peeled off from the laminate, and the bond strength wasuniform across the sample.

EXAMPLE 5

[0072] Composite foils produced by the method of Example 1 werelaminated on both surfaces of an inner layer material on which wiringpatterns previously had been formed, using a 0.18 mm thick FR-4 prepregas an intermediate layer and by pressing at 175° C. and 25 kg/cm² for 60minutes. After cooling down, the bond between the ultra-thin copper foiland the metallic carrier was measured to be 0.015 kgf/cm. The supportingmetallic layer could easily be separated from the laminate, and the bondstrength was uniform.

EXAMPLE 6

[0073] Using a copper-clad laminate produced by the methods of Example1, a drill having a diameter of 0.3 mm was used to make holes in thelaminate. A conventional desmear treatment with potassium permanganatesolution was done to remove epoxy resin. Then, the panel was plated to athickness of 15 μm. A circuit pattern was formed (line width/linespace=50 μm/50 μm) to obtain a printed wiring board. The wiring patternwas found to be free of shorts or open spaces. Two of these printedwiring boards were laminated together using a 0.18 mm thick FR-4 prepregby hot pressing to obtain a multi-layer printed wiring board having fourconductive layers. There were no problems. The ultra-thin copper foilwas not torn and no wrinkles were formed. Thus, an improved multi-layerprinted wiring board was obtained in both Examples 5 and 6, which hadvery fine 50 μm circuit lines and spaces.

EXAMPLE 7

[0074] The ultra-thin copper foil in each of the copper-clad laminatesmade in Examples 1-5 was copper-plated by electroplating to a totalthickness of 18 μm. Next, the peel strength (measured by JIS-C-6481)between the 18 μm thick copper layer and substrate was measured. Theultra-thin copper cannot be tested by this method as it is too thin andweak, thus, the additional copper plating is applied to permit the testto be done.

[0075] A circuit pattern was formed by conventional etching methodsusing acid copper chloride and dry film resist on the five copper-cladlaminates from Examples 1-5. The line width/line spacing again was 50μm/50 μm. The results obtained are shown in Table 1. TABLE 1 PeelStrength (kgf/cm) Etching Properties Example 1 1.3 Good Example 2 1.2Good Example 3 1.3 Good Example 4 1.0 Good Example 5 1.3 Good

[0076] The above results show suitable performance of the copper-cladlaminates for electronic equipment. It can be concluded that the peelstrength of the ultra-thin copper foil is comparable to conventionalthick copper foils, and much greater than the bond strength measuredbetween the ultra-thin copper and the carrier layer. Furthermore, theselaminates were found to yield very superior circuit patterns in thatthey had no opens or shorts at a line width and spacing of 50 μm.

[0077] As demonstrated above, the invention provides an organic releaselayer between a metallic carrier and an ultra-thin copper foil. The bondstrength is uniform and low enough so that the ultra-thin copper foil isnot damaged when the metal carrier layer is stripped off. Thus, anultra-thin copper foil having improved handling properties has beenproduced. The organic release layer can be very easily formed by dippingor coating an aqueous solution containing triazoles, thiazoles,imidazoles, or their derivatives.

[0078] The release layer is not inorganic, and only a thin organic layerremains on the ultra-thin copper foil after separating the metal carrierlayer. Thus, only cleaning with dilute acid to remove the organicresidue is needed. Furthermore, since no metal is used in the releaselayer, the supporting metallic layer can be recycled and waste solutiondisposal is easy, without causing environmental problems.

What is claimed is:
 1. A composite foil comprising a release layeruniformly disposed between a metal carrier layer and an ultra-thincopper foil wherein the release layer is an organic compoundcharacterized by: a. containing at least one nitrogen atom; b. beingcapable of forming a bond with said copper foil and said metal carrierlayer; c. retaining the ability to bond with copper after exposure totemperatures used in laminating said composite foil to a substrate; d.having a bond strength to copper which is uniform and lower than thepeel strength of said copper foil when laminated to a substrate, therebypermitting the metal carrier to be separated from said copper foil whensaid copper foil has been laminated to a substrate, and e. providing avery thin release layer which permits uniform deposition of saidultra-thin copper foil.
 2. A composite foil of claim 1, wherein saidrelease layer comprises a heterocyclic compound and at least one memberof the group consisting of triazoles, thiazoles, imidazoles, andderivatives thereof.
 3. A composite foil of claim 1, wherein saidrelease layer is a triazole or derivatives thereof.
 4. A composite foilof claim 3, wherein said triazole is CBTA, CBTA-U, BTA, or mixturesthereof.
 5. A composite foil of claim 1, wherein said organic compoundretains its ability to bond with copper after exposure to temperaturesabove 120° C.
 6. A composite foil of claim 1, wherein the bond strengthbetween said ultra-thin copper foil and said metal carrier layer isabout 0.005-0.3 kgf/cm.
 7. A composite foil of claim 1, wherein saidmetal carrier layer is copper or copper alloy.
 8. A composite foil ofclaim 1, wherein said metal carrier layer is copper-coated aluminum. 9.A composite foil of claim 1, wherein the thickness of said ultra-thincopper foil is less than 12 μm.
 10. A composite foil of claim 1, whereinsaid metal carrier layer has a thickness up to about 5 mm.
 11. Acomposite foil of claim 10, wherein said metal carrier layer has athickness of about 18-70 μm.
 12. A composite foil of claim 1, whereinsaid ultra-thin copper foil further comprises a nodularization treatmenton the exposed copper surface thereof to enhance the peel strengthbetween the ultra-thin copper foil and a substrate.
 13. A composite foilof claim 12, wherein said ultra-thin copper foil further comprises apassivated treatment on the exposed surface thereof to prevent oxidationof the surface of the ultra-thin copper foil.
 14. A copper-clad laminatecomprising a composite foil of claim 1 laminated to a substrate.
 15. Acopper-clad laminate of claim 14, wherein the metal carrier layer hasbeen removed to expose said ultra-thin copper foil.
 16. A printed wiringboard comprising the copper-clad laminate of claim
 15. 17. A multi-layerprinting wiring board formed by laminating on at least one side of aninner layer board on which wiring patterns were previously formed acomposite foil according to claim 1 to obtain a copper clad laminate,separated from the metal carrier layer from the copper clad laminate toexpose the ultra-thin copper foil, and forming a wiring pattern on saidultra-thin copper foil, thereby obtaining a multi-layer printed wiringboards.
 18. A multi-layer printed wiring board formed by laminating aplurality of printed wiring boards according to claim
 16. 19. Amulti-layer printed wiring board formed by laminating a plurality ofprinted wiring boards according to claim
 17. 20. A process for producinga composite foil comprising: a. uniformly applying an organic releaselayer to a metal carrier layer; and b. depositing an ultra-thin copperlayer on said organic release layer; wherein said organic release layeris an organic compound characterized by:
 1. containing at least onenitrogen atom;
 2. being capable of forming a bond with said copper layerand said metal carrier layer;
 3. retaining the ability to bond withcopper after exposure to temperatures used in laminating said compositefoil to a substrate;
 4. having a bond strength to copper which isuniform and lower than the peel strength of said copper foil whenlaminated to a substrate, thereby permitting the metal carrier layer tobe separated from said copper foil when said copper foil has beenlaminated to a substrate, and
 5. providing a very thin release layerwhich permits uniform deposition of said ultra-thin copper foil.
 21. Aprocess of claim 20, wherein said organic release layer is applied tosaid metal carrier layer by dipping said metal carrier layer in anaqueous solution of said organic compound and thereafter rinsing saidmetal carrier layer in water, leaving a thin layer of said organiccompound bonded to said metal carrier layer.
 22. A process of claim 20,wherein said organic compound is a heterocyclic compound selected fromthe group consisting of triazoles, thiazoles, imidazoles, andderivatives thereof.
 23. A process of claim 22, wherein said organiccompound is a triazole or derivatives thereof.
 24. A process of claim23, wherein said triazole is CBTA, CBTA-U, BTA, or mixtures thereof. 25.A process of claim 20, wherein said organic compound retains its abilityto bond to copper after exposure to temperatures above 120° C.
 26. Aprocess of claim 20, wherein said ultra-thin copper layer is depositedon said organic release layer by electrodeposition.
 27. A process ofclaim 26, wherein said electrodeposition employs an electrolytic bathwhich is substantially free of acid.
 28. A process of claim 27, whereinsaid electrolytic bath is a copper cyanide or copper pyrophosphate bath.29. A process of claim 27, wherein said electrodeposition is used todeposit at least 0.5 μm of copper on said organic release layer.
 30. Aprocess of claim 26, wherein said electrodeposition employs a firstelectrolytic bath containing copper pyrophosphate to deposit a firstlayer of copper, followed by further electrodeposition using a secondelectrolytic bath containing copper sulfate and sulfuric acid to deposita second layer of copper on said first layer.
 31. A process of claim 29,wherein said first layer is at least 0.50 μm thick and the thickness ofsaid first and second layers is up to about 12 μm.
 32. A process ofclaim 20, further comprising the step of applying to said ultra-thincopper layer a nodularizing treatment to improve adhesion of saidultra-thin copper layer to a substrate.
 33. A process of claim 32,further comprising the step of applying a passivation treatment to saidnodularized ultra-thin copper layer to prevent oxidation of saidultra-thin copper layer. 34 A process of claim 33, wherein saidpassivation treatment comprises depositing at least one member of thegroup consisting of zinc, zinc chromate, nickel, tin, cobalt andchromium to said nodularized ultra-thin copper layer.